Maybe your timing cover is warped. I have never had a leak there and I have built and owned 3 428's and now a 427.
I use the black silicone sealer and I put the sealer on the timing cover then put a few bolts through to align the gasket then I place the gasket on the cover and I wait maybe an hour before I attach the cover to the engine with more silicone. I assume your probably leaking at the
oil pan area so use the silicone on the bottom of your timing cover too.
Clois